In general the present invention relates to telephone station equipment, and more particularly to multiple telephone set installations serviced by multiple telephone lines from a central office.
Many telephone customers, especially businesses, need a telephone installation in which calls can be placed or received at any one of a number of telephone sets, and over any one of a plurality of available telephone lines from a central office. Such installations are in general available and are called key telephone systems (or KTS). One of the most commonly used systems of this type is intended for meeting the needs of customers that require a relatively large number of telephone lines, such as five or more lines, and for these customers the cost of the service is not unreasonable on a per line basis. However, there is a need for a less complex, lower cost key telephone system capable of efficiently meeting the needs of customers that have phone traffic requiring less than five lines but more than one line. For example, it is believed that there are many customers, in small businesses for example, that have a definite need for two line, key telephone service, but cannot justify the cost of such service because most available key telephone systems are cost efficient when used with five or more telephone lines.
One of the reasons for the prohibitively high cost of key telephone systems when used for two line installations is that the system includes a central control unit that must be set up and stored at a location remote from the telephone sets. The central control unit in turn involves a substantial installation cost, both for the control unit itself and also for the auxiliary wiring that must be strung between the control unit and each of the multiple telephone sets. Also, the telephone sets themselves must be specially made to accommodate the maximum number of key functions for which the system has been designed. For example, a standard KTS desk set may provide for five or more line select buttons, even though the customer is only using the system for two lines and thus only two of the available buttons are functional.
Although other telephone systems are available for providing a form of two line service on a relatively low cost basis, none of these existing two line systems have the capability or offer the convenience and flexibility of key telephone systems.
Accordingly, an overall object of the invention is to provide a novel key telephone system capable of being installed at a relatively low cost.
A more particular object of the invention is to provide a hold control for use in a key telephone system, that is capable of placing a line in a hold condition while a telephone set is temporarily disconnected from the line and, is capable of generating a hold indicating signal on the line for indicating to the other stations in the system that the line is on "hold". A related object is to provide such a hold control circuit in which the placing of the line in the hold condition, and the generating of the hold indicating signal on the line do not interfere with the normal supervisory signalling between the central office and the telephone sets.
Still another object of the invention is to provide such a hold control which has the further characteristic of providing reliable and consistent operation in key telephone systems installed at widely varying line distances from the central office.